Various systems for detecting the liquid level in a storage tank have been utilized in the past. In one measuring system a capacitance probe is used to measure the liquid level in storage tanks. The capacitance probe relies upon the dielectric difference between the liquid and air in the storage tank. The capacitance probe is mounted vertically in the storage tank and the liquid level (and consequently the quantity of liquid in the tank) is periodically measured by a processor connected to the capacitance probe. Such capacitance systems commonly employ multiple probes, wired in parallel, particularly in large or irregularly-shaped tanks. It can be difficult to determine the contribution of each probe to the overall liquid level measurement. One solution is to individually wire each probe to its associated electronics. However, this can increase the cost and weight of the system.
Another measuring system works upon the principle of transmitting an ultrasonic signal from a signal source through the liquid. For example, FIG. 1 is a diagram of a tank 10 and a sensing portion 12 of a known multiple-probe ultrasonic level gauge system. The sensing portion 12 includes a number of probes 14a-14d oriented perpendicular with respect to the bottom surface 18 of tank. Each probe 14a-14d includes a stillwell 16a-16d and an ultrasonic transducer 20a-20d, respectively, located at the bottom of each stillwell. A remote processing system (not shown) separately excites and receives signals from each transducer 20a-20d over a dedicated wire pair 22a-22d. To sense the level 24 of liquid 26 in the tank 10 a drive pulse is transmitted to each transducer 20a-20d over an associated wire pair 22a-22d causing each transducer to generate an ultrasonic signal which propagates from the transducer 20 toward the surface 24 of liquid 26. The ultrasonic signal reflects off of the interface of the liquid 26 and air 28 which is the surface 24 of the liquid and propagates back to the transducer 20. Knowing the speed of sound through the particular liquid 26 and the measured time for the ultrasonic signal to propagate from a transducer 20 to the surface 24 of liquid 26 and back, the depth of the liquid in storage tank 10 can be calculated.
While such a system is able to differentiate signals from the separate transducers by virtue of the dedicated wire pairs, the overhead of routing the individual wiring from each transducer to the remote processing system can again add to the cost and weight of the liquid level measuring system. In aviation-type fuel tanks the weight of a liquid level gauging system can be an important design constraint.
In view of the above mentioned shortcomings of conventional liquid level measuring systems, it is believed that there is a demand in the market for an ultrasonic system which is lightweight and inexpensive, and which permits mapping of the liquid level in the tank.